1. Field of the Invention
This invention is related in general to apparatus for measuring angles. In particular, the invention consists of a novel device for calibrating autocollimators.
2. Description of the Related Art
Autocollimators are optical instruments used to measure angles. For instance, for quality control purposes it is often necessary to know the exact angle of a flat surface with respect to a reference plane, such as the surface of a slider in a read/write computer drive head. The part to be tested is placed on a sample stage or mounting fixture, which is illuminated by a light source in the autocollimator, and the angle is determined by the position of the reflected light focused by a lens on a light detector.
A typical autocollimator system 10, well known in the art, is illustrated in FIG. 1, where a light source 12 produces a beam L of collimated light that is partially reflected by a beamsplitter 14 toward a reflective test surface or mirror S. The return light reflected from the surface S is passed through the beamsplitter and focussed on the surface of a detector 16 by a focussing lens 18. A variable attenuator 20 may also be used, if necessary.
Thus, the autocollimator 10 is employed to measure the angle of the reflective surface S with respect to a reference plane. In practice, the mirror surface S serves as sample stage, or is coplanar to it, and provides the reference plane with respect to which a test surface is measured. As those skilled in the art readily understand, there is direct relationship between the angle of the test surface S and the location of the point on the detector 16 where the reflected light is focussed. For small angles (less than about 5 degrees) the relationship is substantially linear. This relationship is normally referred to as the scaling factor of the instrument, which is used to determine the angle of the tested surface as a function of the position of the reflected light on the detector. Since the scaling factor is affected by environmental conditions that may cause slight deviations from the rated performance, autocollimators require periodic calibration. The preferable practice in the art is to calibrate the instrument periodically, often before each measurement session.
FIG. 1 illustrates the shift of an image focussed on the detector 16 as a result of an angle introduced in the sample surface S. For the purposes of this disclosure, the x and y directions are taken to define a horizontal plane substantially perpendicular to the light incident to the reflective surface S and the z direction is the vertical dimension normal to that plane. An initial position P1 of the surface S is shown in solid line and a calibration position P2, after the surface S has been tilted by a known angle .phi., is shown in broken line. In practice, the angle .phi. in the test surface is provided by the introduction of a high-precision wedge 30 or sine bar placed on the sample stage of the instrument for the purpose of calibration, as illustrated in FIG. 2. These devices are commonly used in the art and are well known to users of autocollimators; therefore, they will not be described here.
FIGS. 1 and 2 are two-dimensional representations (in the x,z plane) of the light-path change introduced by a tilt of the reflective surface S with respect to the x axis. Obviously, calibration requires measurements with respect to angles introduced in at least two orthogonal directions (x,y measurements). Therefore, the same procedure must be repeated for an angle introduced with respect to the y axis and a total of at least four data points and corresponding measurements are required to calculate a scaling factor for both orthogonal coordinates. Assuming for simplicity of description that the sample surface S in its initial position P1 is perpendicular to the incident light beam LI, the reflected beam is coincident with the incident beam (that is, it is reflected with a 0 degree angle of reflection) and travels back through the beamsplitter 14 and the focusing lens 18 to form a high-intensity spot at a point 22 on the detector 16. When the angle .phi. is introduced in the sample surface S, the incident light LI is reflected at an angle .alpha.=2.phi.. The reflected beam LR travels back through the beamsplitter 14 and the focusing lens 18 and forms a spot at a point 24 on the detector 16 a distance d from the original point 22. As those skilled in the art would readily recognized, for an autocollimator system having focal length f, the shift d is independent of the vertical position of the test surface and is equal to the product (f)(tan.phi.). For a relatively large focal length f in comparison to the distance d (which results from small angles .phi.), the linear relationship d.apprxeq.f.phi. is substantially true. Thus, an x scaling factor is readily established by this equation for the x direction. By repeating the measurements with a tilt in the y direction (normal to the first set of measurements), a y scaling factor is similarly determined, thereby providing the necessary information for calibration. A computer coupled to the autocollimator system can be used to perform the necessary calculations in a known manner.
The prior-art procedure for calibrating autocollimators suffers from several drawbacks that affect its desirability and reliability. The precision of calibration is completely dependent on the quality of the wedges or sine-bar devices utilized to introduce an angle in the test surface; any imperfection in the flatness of the surfaces may cause errors in the calculation of the scaling factors, which in turn would result in imperfect angle measurements. The use of either device requires its placement in intimate contact with the flat surface of the sample stage in the autocollimator; given the extreme precision needed for a good measurement, dirt, dust, or even fingerprints may prevent good surface contact and produce erroneous calibration measurements. The sample stage of autocollimators is typically very small, in the order of centimeters, while commercial wedges and sine bars are materially larger and require some adaptation for use in calibrating autocollimators. At least two measurements are required for calibration in each orthogonal direction, for a total of four measurements, which typically take 5 to 30 minutes to accomplish, a substantial amount of time. Finally, the accuracy of calibration is affected by temperature changes in the gauge blocks, which can occur as a result of being handled by an operator carrying out the procedure.
It is noted that autocollimators are manufactured according to specific design parameters that provide corresponding design scaling factors. If all the physical parameters of the instrument were known exactly and were unaffected by environmental conditions, the scaling factors could be calculated precisely from those parameters. In practice, though, the design scaling factors provide only an approximation that is not suitable for the level of precision typically required by users of autocollimators. Therefore, there is still a need for an improved method and device for calibrating conventional autocollimators.